Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art nor that such background art is widely known or forms part of the common general knowledge in the field in Australia or worldwide.
All references, including any patents or patent applications, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of background publications are referred to herein, this reference does not constitute an admission that any of these documents are prior art, nor that they form part of the common general knowledge in the art, in Australia or in any other country.
Conventional voltage source DC/AC inverter with a typical LCL filter can only handle low power at low voltage applications. To increase power handling capability at high voltage, new circuits need be developed.
Furthermore to operate a conventional grid-tied voltage source DC/AC inverter with an LCL filter at relatively high power, the series inductance of the filter used with the inverter must be kept small to avoid excessive fundamental voltage drop across it. Due to this, harmonic components of the current into the grid at a lower percent of rated power could be more than its fundamental component. Moreover, such circuits with small series impedance could produce excessively high short-circuit currents which have the potential to damage other parts of the circuit. When such a converter is used as a grid-forming generator in a microgrid, it could lose its capability to take its grid-forming role when its operating power level is only a small fraction of its rated power. Hence new circuit topologies need be developed for both general purpose and microgrid applications.
A microgrid is an electrical system that includes multiple loads and distributed energy resources (DER) that can be operated in parallel with the broader utility grid or as an electrical island. It is difficult to manage electricity availability across the microgrid since renewable energy sources which often form part of the microgrid typically are inherently transient (e.g. solar power generation) and not often predictable (e.g., wind power generation).
An islanded microgrid is a microgrid that is separated from the main power grid in case of disturbances outside the microgrid.
Due to the drastic power generation variances inherent in renewable energy sources, the grid-forming generators and grid-supporting generators in the microgrid need to be very adaptive. Currently conventional circuit topologies and their control methods do not work properly to suit such sudden changes. Furthermore the traditional voltage and frequency droop based microgrid management method has drawbacks such as a) poor transient performance or instability issues due to the use of average values of active and reactive power over a cycle; b) ignoring load dynamics that can result in failure subsequent to large and/or rapid load changes; c) inability to narrow down the electrical supply frequency within certain limits independent of system loading conditions etc.
The present invention overcomes or ameliorates at least one or more of the disadvantages of the prior art, or to provide a useful alternative. The invention further provides systems and methods for operating a microgrid at high power levels, for example up to tens of Mega-Watts or higher.
Disclosed herein are applications of new LCCL or LCC filters to voltage source DC/AC converters or modified forms thereof of converters for general purpose usage and also for microgrid operation and management. The converters, either rectifiers or inverters can be single-stage or multi-stage for both single phase, three phase and other multiple phase applications.
There is a demand for applying the circuits disclosed herein as forming components in a microgrid and incorporating them with invented control methods to overcome the drawbacks in the design and operation of a microgrid.